The invention is related to a process of modifying a color image tube deviator, especially a deviator associated with a tube comprising three coplanar guns, wherein the combination of tube and deviator assembly is autoconvergent. The deviator, while retaining its autoconvergent property, may be associated with a family of tubes having different dimensions but the same deflection angle.
It is known that a color television image tube comprises three electron guns each one of which is adapted to excite on the screen a luminescent substance of a given color, i.e. red, green or blue. At every moment these three beams impact with a triplet of three luminescent substances on the screen. The displacement or shifting of these three electron beams is achieved by means of a deviator mounted on the neck of the tube and comprising two coils through each one of which flows an alternating electric current generating a variable magnetic field for deviating said electron beams. One of said coils provides for the horizontal sweep, i.e. line sweep, of the screen, while the other coil ensures the vertical sweep or field sweep.
It is easily understood that the design and adjustment of the coils or windings of a deviator are difficult operations, since the three electron beams must converge on the screen at every point thereof, and each beam must be focalized on the center of the screen, while the tube has not a rotationally symetric shape, the screen having substantially flat or planar configuration, and the guns producing beams located in one common plane. Thus, with a view to adjusting an autoconvergent tube plus deviator assembly, a compromise has to be established between a certain number of contradictory requirements. Therefore a deviator designed for a tube of a given type and having given dimensions will not be convenient for use with a tube of a different type and/or having different dimensions.
However it has already been noted that one given deviator may be used in association with two autoconvergent tubes which differ from each other only in the size of the screen (and thus also in the length of the conical portion), provided that the distance between the line deviation center and the field deviation center is modified with a view to reducing the anisotropic astigmatism (cause of deficiency of convergence between horizontal lines). For achieving such modification, it has been current practice up to now to displace axially one coil with respect to the other, or to vary the length of each coil.